This invention relates to mineral fibre products, and methods of making them, which are of particular value for use as heat insulation and fire protection at high temperatures, for instance above 700° C. and often above 900° C. or above 1000° C. In particular, it relates to such products formed from mineral fibres which are based on a silicate network which comprises silicon, calcium, magnesium, iron, aluminium and oxygen atoms and optionally small amounts of alkali metal and other minor components.
Typically the fibres to which the invention relates always include at least 3% FeO and at least 5% MgO and never more than 8% alkali metal oxide. The amount of CaO is usually at least 8% and the amount of SiO2 is usually 35 to 55%, and the amount of Al2O3 is usually up to 25%. Any other elements are usually present in only very small amounts, for instance below 5%, and usually below 2%, of the oxides.
In this specification all analytical amounts are expressed as percentages by weight (unless otherwise specified) of oxide based on all the elements expressed as oxides. The iron is always expressed herein as FeO even though, in practice, some or most of it may be present as ferric.
These fibres are therefore the fibres of the types known generally by terms such as rock wool, slag wool or stone wool. They are different, as regards their analysis and their properties and their manufacture, from those generally known as glass fibres and which have a high alkali metal oxide content (typically above 15 or 20%) and which may also contain a significant boron content, typically above 5% B2O3, and which are always substantially free of iron. It is essential in drawn (and optionally flame attenuated) glass fibres to minimise iron because the presence of iron in amounts greater than trace quantities, for instance 1%, significantly alters the colour of the fibres and this can significantly influence the methods of heating and extruding the melt as filaments.
It is known that the properties of all silicate fibres, including the fibres to which the invention relates, depend in part on the chemical analysis of the fibres and thus on the nature of the total silicate network including atoms retained with in the network. It is also known that when the temperature of an assembly of the fibres exceeds Tg (the glass transition temperature) the fibres adopt a visco-elastic state with the result that they may tend to start to lose their individual fibrous form, and in particular there may be some flow at the surfaces of the fibres, leading to fusion at the intersections of fibres. At higher temperatures, the flow becomes more serious and the fibres may fuse into a much smaller volume than was occupied by the original assembly of fibres. At Tc (the crystallisation temperature) the tetrahedral network will tend to reorganise into a crystalline structure. As the temperatures increase still further, the crystals will start to melt at Tm, the melting temperature. For a typical rock fibre Tg may be around 650-700° C. eg 680° C., Tc around 820-900° C. eg 850° C. and Tm around 1,000 to 1,100° C. eg 1050° C.
It is also known that the conditions under which the fibre product is exposed to increasing temperature may significantly influence the performance of the fibres. In particular it is known that when a thick slab of bonded, high density, mineral wool is exposed to high temperatures, the core may shrink due to sintering to leave a void (with the result that the fire protection properties are unsatisfactory) whilst the outer surface may remain reasonably fibrous.
A particular problem arises with fire doors and sandwich panels, namely semi-sealed or sealed products containing bonded mineral wool between sheets of material which prevent free access of air. It is found that these may fail due to sintering at a temperature lower than would be expected having regard to the known failure temperatures of the mineral wool.
These failures of bonded mineral wool and of sealed or semi-sealed products have generally been assumed to be caused by the exotherm created by the combustion of organic bonding agent in the mineral fibre product, this exotherm giving local heating so as to cause localised increase in temperature, and therefore sintering.
The mechanisms by which the fibrous assembly eventually shrinks, for instance to form a void, as the temperature increases can therefore be seen to be rather complex but, whatever the mechanism, the effect is generally referred to as sintering.
It is essential that a fire door or other heat insulation or fire protection assembly should reliably retain its performance properties. It is therefore essential that shrinkage or sintering does not occur since, if there is shrinkage or sintering, there will be failure in the insulation and fire protection properties in those regions where shrinkage and sintering has occurred. It is therefore conventional to report the temperature at which any particular type of fibre is likely to fail. For instance shrinkage or sintering temperatures are commonly quoted as an indication of the temperature at which the fibrous nature of the fibres is likely to be lost under defined conditions. Numerous publications therefore quote collapse or sintering temperatures.
This invention relates particularly to enhancing the fibres so as to give improved resistance to shrinkage or sintering, for instance so as to increase the temperature at which shrinkage or sintering may occur.
At present, the prior art proposes two main ways of achieving this.
One way of improving resistance to shrinkage or sintering has been to add various endothermic compounds into mineral fibre products which are intended for fire protection, so that the added compound will tend to absorb heat energy and thus delay the onset of shrinkage or sintering, despite the external temperature being higher than the fibre might have been able to with stand in the absence of the added material. A typical example is EP-A-0936060. Instead of using strictly endothermic materials, it is also known to use reactive materials, for instance as in GB-A-1,281,381.
Another way is to select appropriately the chemical analysis of the melt from which the fibres are formed, for instance by forming them from a melt having high amounts of iron and/or magnesium and/or aluminium in the melt and therefore in the silicate network. The ultimate would be to make ceramic fibres, for instance using melts containing high amounts of alumina, typically above 30%. An example of such fibres which are, or are almost, ceramic is given in U.S. Pat. No. 5,312,806. Unfortunately forming fibres which are ceramic or near ceramic as a result of high alumina contents, and the subsequent handling of the fibres, is difficult and expensive.
The selection of the chemical content of mineral fibres now has to take account of numerous factors including ability to form the fibres by conventional techniques, cost and availability of raw materials, biodegradability of the fibres, and weather resistance of the fibres and, as discussed above, the sintering properties of the fibres. Accordingly it is undesirable to be restricted by the additional requirement of improving resistance to shrinkage and sintering.
The problem to be solved by the invention therefore is to find a way of improving the resistance to shrinkage and/or sintering of fibrous products made from a wide range of convenient rock, stone and slag melts, so as to reduce or eliminate the need for total reformulation of the melt (for instance high aluminium) and with out the compulsion to add endothermic compounds to the fibrous product.